Changes In Dielectric Function Research Articles

Long-Lived Photo-Response of Multi-Layer N-Doped Graphene-Based Films.

New insights into the mechanism of the improved photo(electro)catalytic activity of graphene by heteroatom doping were explored by transient transmittance and reflectance spectroscopy of multi-layer N-doped graphene-based samples on a quartz substrate prepared by chitosan pyrolysis in the temperature range 900-1200 °C compared to an undoped graphene control. All samples had an expected photo-response: fast relaxation (within 1 ps) due to decreased plasmon damping and increased conductivity. However, the N-doped graphenes had an additional transient absorption signal of roughly 10 times lower intensity, with 10-50 ps formation time and the lifetime extending into the nanosecond domain. These photo-induced responses were recalculated as (complex) dielectric function changes and decomposed into Drude-Lorentz parameters to derive the origin of the opto(electronic) responses. Consequently, the long-lived responses were revealed to have different dielectric function spectra from those of the short-lived responses, which was ultimately attributed to electron trapping at doping centers. These trapped electrons are presumed to be responsible for the improved catalytic activity of multi-layer N-doped graphene-based films compared to that of multi-layer undoped graphene-based films.

Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation

The photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed.

Generation of terahertz radiation by a Hermite–Gaussian laser beam inside magnetoplasma with a density ramp

In the present scheme of work, the Hermite–Gaussian (HG) laser beam dynamics has been investigated under the influence of an upward density ramp inside magnetized plasma, where both relativistic and ponderomotive nonlinearities are operative. One can achieve self-focusing of laser beam due to the change in the medium's dielectric function, which comes into operation due to the expulsion of plasma electrons from the high intensity to the low-intensity region by ponderomotive force and their motion at relativistic speeds. The dynamics of the laser beam and terahertz generation have been investigated by using the moment theory approach. It has been observed from the present analysis that the dynamics of the laser beam and the production of terahertz radiations strongly depends upon the HG laser beam and plasma parameters. In addition to this, the effect of density ramp and magnetic field has also been investigated on the efficiency of terahertz generation. It has been observed that higher-order modes of the HG laser beam play a dominant role in the production of terahertz radiations.

Temperature dependence of the dielectric function of dehydrated biological samples in the THz band.

Terahertz technology has demonstrated enormous potential for the analysis of biological systems and the diagnosis of some medical conditions, given its high sensitivity to detect water content. In previously published papers, effective medium theories are used to extract the water content from the terahertz measurements. When the dielectric functions of water and dehydrated bio-material are well known, the volumetric fraction of water can be left as the only free parameter in those effective medium theory models. While water complex permittivity is very well known, the dielectric functions of dehydrated tissues are normally measured for each individual application. In previous studies, it has been traditionally assumed that, unlike water, the dielectric function of the dehydrated tissues is temperature independent, measuring it only at room temperature. Yet, this is an aspect that has not been discussed and that is relevant in order to get THz technology closer to clinical and in-the-field applications. In this work, we present the characterization of the complex permittivity of dehydrated tissues; each studied at temperatures ranging from 20°C to 36.5°C. We studied samples of different organism classifications to have a wider confirmation of the results. We find that, in each case, the dielectric function changes of dehydrated tissues caused by temperature are smaller than for water across the same temperature interval. Yet, the changes in the dielectric function of the dehydrated tissue are not negligible and should, in many cases, be taken into account for the processing of terahertz signals that interact with biological tissues. While this study gives a first introduction into the probable relevancy of temperature-dependent optical behavior of biological samples, this work only focuses on the experimental proof for this relationship and will, therefore, not give a deeper analysis of how the underlying models have to be modified.

Extracting quantitative dielectric properties from pump-probe spectroscopy

Optical pump-probe spectroscopy is a powerful tool for the study of non-equilibrium electronic dynamics and finds wide applications across a range of fields, from physics and chemistry to material science and biology. However, a shortcoming of conventional pump-probe spectroscopy is that photoinduced changes in transmission, reflection and scattering can simultaneously contribute to the measured differential spectra, leading to ambiguities in assigning the origin of spectral signatures and ruling out quantitative interpretation of the spectra. Ideally, these methods would measure the underlying dielectric function (or the complex refractive index) which would then directly provide quantitative information on the transient excited state dynamics free of these ambiguities. Here we present and test a model independent route to transform differential transmission or reflection spectra, measured via conventional optical pump-probe spectroscopy, to changes in the quantitative transient dielectric function. We benchmark this method against changes in the real refractive index measured using time-resolved Frequency Domain Interferometry in prototypical inorganic and organic semiconductor films. Our methodology can be applied to existing and future pump-probe data sets, allowing for an unambiguous and quantitative characterisation of the transient photoexcited spectra of materials. This in turn will accelerate the adoption of pump-probe spectroscopy as a facile and robust materials characterisation and screening tool.

Study of Dispersion Characteristics of Surface Plasmon Polaritons at Gyrotropic Interfaces

Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface. The excitation of SPP modes strongly depend on the geometric and material parameters, and the frequency. In this paper, we studied the SPP modes at metal/dielectric (isotropic, anisotropic or gyrotropic), metal/semiconductor and semiconductor/dielectric interfaces. The effect of anisotropy and gyrotropy is considered for the dispersion characteristics. The magnetised semiconductor medium provides an opportunity to make it suitable for sensing and switching applications..

Geometric, Electronic and Optical Properties of Pt-Doped C3N Monolayer Upon NOxAdsorption: A DFT Study

C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N monolayer is reported with comparable and even more desirable sensing behavior upon small gas molecules in comparison to graphene. In this work, we proposed Pt-doped C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N monolayer as a desirable 2D sensing nanomaterials for NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> detection by DFT method. The Pt atom tends to be doped on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{B}_{C-C}$ </tex-math></inline-formula> site of C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N surface, making little effect on the bandgap but changing the indirect semiconducting property on the other hand. Pt-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N monolayer behaves more admirable performance upon NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> adsorption than NO, but both systems are identified as chemisorption with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E_{ad}$ </tex-math></inline-formula> of −2.25 and −1.99 eV, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{T}$ </tex-math></inline-formula> of −0.344 and −0.083 e, respectively. The metallic property is determined in both systems given the calculated zero bandgap, which will increase the electrical conductivity of Pt-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N monolayer largely after adsorption of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> gases. This is the basic sensing mechanism for Pt-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N monolayer upon NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> detection. In the meanwhile, the desirable changes of WF and dielectric function in NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> systems verify the potential of Pt-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N monolayer for NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> detection through field effect transit or optical devices. Our calculations could be meaningful to exploit novel 2D nanomaterial for sensing NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> in order to monitor toxic gases in our environment.

Electronic structure and optical properties of two-dimensional antimony: A first principle study

Two-dimensional (2D) antimonene has similar physical and chemical properties as graphene, BP, MoS2 and other typical 2D materials. It has shown good application prospects in many fields such as nano-optoelectronics, biomedicine and attracted wide attention of the scientific community. In this paper, the crystal structure, electronic structure and optical properties of antimony with few-layers have been systematically studied by the first-principles method based on density functional theory. The results show that the bond lengths of antimonene in few-layers are shorter than those of bulk except monolayer, and the bond lengths and bond angles are smaller with the decrease of layers. In addition, with the increase of the number of layers, the antimonenes are transited from semiconductor to conductor, in which the monolayer is a semiconductor with indirect band-gap, the two and three layers are transitional states, and the four layers are completely transitional to metal state. This conclusion has been proven by PBE method, hybrid HSE06 method and van der Waal’s correction. It is also consistent with the results of density of states. The dielectric function analysis of the 2D antimonenes shows that, with the increase of layers, the polarization ability of antimonenes is stronger, and the static dielectric constant is larger. The imaginary part of dielectric function changes with the change of energy, and with the increase of the number of layers, the curve red-shifts and the frontal value increases. Overall, in the low and high energy regions of photons, with the increase of layer number, the reflection coefficient of antimonene is relatively larger, and in the middle region, the phenomenon of relative aggregation occurs. In addition, the absorption curve corresponds to the imaginary part curve of dielectric function. With the increase of the layer number, the red shift also occurs. For long wavelength light, the fewer number of layers, the worse absorption. But for the ultraviolet region, they all have good absorption. This conclusion is helpful for us to understand the physical and photoelectric properties of two-dimensional antimonene with few layers more comprehensively, and provides a good theoretical basis for the application and research of antimony photoelectric materials and devices.

The (De)confinement Transition in Tachyonic Matter at Finite Temperature

We present a QCD motivated model that mimics QCD theory. We examine the characteristics of the gauge field coupled with the color dielectric function (G) in the presence of temperature (T). The aim is to achieve confinement at low temperatures T&lt;Tc, (Tc is the critical temperature), similar to what occurs among quarks and gluons in hadrons at low energies. Also, we investigate scalar glueballs and QCD string tension and effect of temperature on them. To achieve this, we use the phenomenon of color dielectric function in gauge fields in a slowly varying tachyon medium. This method is suitable for analytically computing the resulting potential, glueball masses, and the string tension associated with the confinement at a finite temperature. We demonstrate that the color dielectric function changes Maxwell’s equation as a function of the tachyon fields and induces the electric field in a way that brings about confinement during the tachyon condensation below the critical temperature.

Measuring the plasma-wall charge by infrared spectroscopy

We show that the charge accumulated by a dielectric plasma-facing solid can be measured by infrared spectroscopy. The approach utilizes a stack of materials supporting a surface plasmon resonance in the infrared. For frequencies near the Berreman resonance of the layer facing the plasma the reflectivity dip —measured from the back of the stack, not in contact with the plasma— depends strongly on the angle of incidence making it an ideal sensor for the changes of the layer's dielectric function due to the polarizability of the trapped surplus charges. The charge-induced shifts of the dip, both as a function of the angle and the frequency of the incident infrared light, are large enough to be measurable by attenuated total reflection setups.

Simulation of Surface Plasmon Resonance on Different Size of a Single Gold Nanoparticle

In this work, the surface plasmon resonance wavelength position is calculated using metal nanoparticle boundary element method (MNPBEM) toolbox from Matlab classes. The surface plasmon resonance wavelength position is determined by full Maxwell’s equations. We present a simulation optical properties study of the dependence of the plasmon resonance spectrum of individual gold nanoparticles on the dielectric function and size change in the extinction efficiency spectrum. Previous studies have shown that the shift of the gold nanoparticle plasmon resonance wavelength increased when the size of gold nanoparticles was increased. However, the dielectric function of gold nanoparticle showed a different shifting when using a higher spectral range. In this work, the attention is focused on the size effects of gold nanoparticle on the position of the plasmon resonance wavelength. The aim of this work is to study the behaviour of the resonance position for different diameter size of gold nanoparticle by using boundary element method simulations. The gold nanoparticle diameter between 10 nm to 100 nm is used in determine the shifting of the surface plasmon resonance. The results in this work indicated that the position of the plasmon resonance wavelength for gold nanoparticle was redshift when the size of gold nanoparticle was increased. However, dielectric function measured by previous experiments show that the high spectral resolution gives the significant redshift to the extinction efficiency spectrum.

Adsorption of NOx (x = 1, 2) gas molecule on pristine and B atom embedded γ-graphyne based on first-principles study

First principle study with density functional theory was carried out to investigate the adsorption behavior of nitrogen oxide (NO and NO2) on pristine and B atom embedded γ-graphyne including the adsorption energy, electron transfer, adsorption distance, magnetic moment, band structure, density of states and optical properties. The results indicate that C atom of sp2 hybridization (compared to sp hybridization) is easier to be substituted by B atom with endothermic process. B atom embedded γ-graphyne have notably stronger chemical interactions with NO and NO2 compared to pristine γ-graphyne with larger adsorption energy and greater changes of molecule orbitals. The calculations of optical properties reveal that B atom embedded γ-graphyne has more obvious changes of dielectric function after adsorbed NO or NO2 near 1.5 eV compared to pristine γ-graphyne illustrating that B-graphyne has the prospect to detect nitrogen oxide using not only electrochemical method but also optical method.

The transport and optical sensing properties of MoS2, MoSe2, WS2 and WSe2 semiconducting transition metal dichalcogenides

In this paper, we investigate the transport and optical properties of the monolayer semiconducting transition metal dichalcogenides (STMDs) in the absence and presence of the NH3, NO, NO2, and O2 gas molecules to assess their potentials as gas sensors. The first-principles calculations based on density functional theory indicate that absorption of the O2, NO2, NO gas molecules on the surface of these materials leads to significant changes in their transmission spectrum. Our calculations predict a charge transfer between the adsorbent gas and any of these STMDs. However, the presence of NH3 molecule has little effect on the transport properties of these materials. The results show that when the STMDs are exposed to NO, NO2, and O2 molecules, the dielectric function changes. Therefore, these materials can be employed as the sensing element in an optical gas sensor.

Spectroscopic planar diffraction ellipsometry of Si‐based multilayer structure with subwavelength grating

Spectroscopic ellipsometry has been applied at room temperature to a multilayer device structure with plain and nanograting‐embedded Si layer on the top. In both cases, the real and imaginary parts of the complex dielectric function of the top layer have been retrieved. In nanograting case, ellipsometric data were collected in planar diffraction geometry. A profound change in dielectric function in the photon energy region between 1.5 and 3.5 eV is established for nanograting‐embedded Si layer compared to plain Si layer. Both real and imaginary parts of the former are no longer Si‐like and resemble more those for metals. The maximum of interband density‐of‐states for optical transitions is positioned at 2.1 eV and coincides with the energy of optical transitions from the main irradiative eigenstate. The obtained results are discussed in terms of geometry‐induced doping changes in interband density of states.

Dual-modulated photoreflectance spectra of semi-insulating GaAs

For a semiconductor material, the characterization of its electronic band structure is very important for analyzing its physical properties and applications in semiconductor-based devices. Photoreflectance spectroscopy is a contactless and highly sensitive method of characterizing electronic band structures of semiconductor materials. In the photoreflectance spectroscopy, the modulation of pumping laser can cause a change in material dielectric function particularly around the singularity points of joint density of states. Thus the information about the critical points in electronic band structure can be obtained by measuring these subtle changes. However, in the conventional single-modulated photoreflectance spectroscopy, Rayleigh scattering and inevitable photoluminescence signals originating from the pumping laser strongly disturb the line shape fitting of photoreflectance signal and influence the determination of critical point numbers. Thus, experimental technique of photoreflectance spectroscopy needs further optimizing. In this work, we make some improvements on the basis of traditional measurement technique of photoreflectance spectroscopy. We set an additional optical chopper for the pumping laser which can modulate the amplitude of the photoreflectance signal. We use a dual-channel lock-in amplifier to demodulate both the unmodulated reflectance signals and the subtle changes in modulated reflectance signals at the same time, which avoids the systematic errors derived from multiple measurements compared with the single-modulated photoreflectance measurement. The combination of dual-modulated technique and dual-channel lock-in amplifier can successfully eliminate the disturbances from Rayleigh scattering and photoluminescence, thus improving the signal-to-noise ratio of the system. Under a visible laser (2.33 eV) pumping, we measure the room-temperature dual-modulated photoreflectance spectrum of semi-insulating GaAs in a region from near-infrared to ultraviolet (1.1 ～6.0 eV) and obtain several optical features which correspond to certain critical points in its electronic band structure. Besides the unambiguously resolved energy level transition of E0 and E0+0 around the bandgap, we also obtain several high-energy optical features above the energy of pumping laser which are related to high-energy level transitions of E1, E1+1, E0' and E2 in the electronic band structure of GaAs. This is consistent with the results from ellipsometric spectroscopy and electroreflectance spectroscopy. The results demonstrate that for those high-energy optical features, the mechanism for photoreflectance is that the photon-generated carriers modulate the build-in electric field which affects the overall electronic band structures, rather than the band filling effect around those critical points. This indicates that dual-modulated photoreflectance performs better in the characterization of semiconductors electronic band structure at critical point around and above its bandgap.

Effective method to study the thickness-dependent dielectric functions of nanometal thin film.

A new method for measuring the dielectric functions change with the thickness of nanometal thin films was proposed. To confirm the accuracy and reliability of the method, a nano-thin wedge-shaped gold (Au) film with continuously varied thicknesses was designed and prepared on K9 glass by direct-current-sputtering (DC-sputtering). The thicknesses and the dielectric functions in the wavelength range of 300-1100nm of the nano-thin Au films were obtained by fitting the ellipsometric parameters with the Drude and critical points model. Results show that while the real part of the dielectric function (ϵ1) changes marginally with increasing film thickness, the imaginary part (ϵ2) decreases drastically with the film thickness, approaching a stable value when the film thickness increases up to about 42nm. This method is particularly useful in the study of thickness-dependent optical properties of nano-thin film.

Dynamical charge and pseudospin currents in graphene and possible Cooper pair formation

Based on the quantum kinetic equations for systems with SU(2) structure, regularization-free density and pseudospin currents are calculated in graphene realized as the infinite mass-limit of electrons with quadratic dispersion and a proper spin-orbit coupling. Correspondingly the currents possess no quasiparticle part but only anomalous parts. The intraband and interband conductivities are discussed with respect to magnetic fields and magnetic domain puddles. It is found that the magnetic field and meanfield of domains can be represented by an effective Zeeman field. For large Zeeman fields the dynamical conductivities become independent of the density and are universal in this sense. The different limits of vanishing density, relaxation, frequency, and Zeeman field are not interchangeable. The optical conductivity agrees well with the experimental values using screened impurity scattering and an effective Zeeman field. The universal value of Hall conductivity is shown to be modified due to the Zeeman field. The pseudospin current reveals an anomaly since a quasiparticle part appears though it vanishes for particle currents. The density and pseudospin response functions to an external electric field are calculated and the dielectric function is discussed with respect to collective excitations. A frequency and wave-vector range is identified where the dielectric function changes sign and the repulsive Coulomb potential becomes effectively attractive allowing Cooper pairing.

Gold nano-ripple structure with potential for bio molecular sensing applications

We introduce a simple and cost-effective scheme for bio-sensing using nano-ripple structures. One dimensional metallic nano-ripple structures formed by gas cluster ion beam irradiation have shown polarization of light as well as localized surface plasmon resonance. These localized surface plasmon resonance based bio sensors not only are capable of label free real time analytical detection but also show high sensitivity. The nano-surface morphology determines the changes in the plasmonic properties of nano-structures hence the plasmonic response is tunable. By adsorbing a mono-layer of thiolated organic compound on the surface of these substrates we identified the shift in the localized surface plasmon resonance peaks triggered by the change of dielectric function in the neighborhood of the structures. These plasmonic nano-metallic structures can be utilized to observe the change of localized surface plasmon resonance frequency due to the cycle of adsorption, re-adsorption and reactions taking place on the surface that can potentially be mapped in to reaction mechanics. The bio-sensor has monolayer molecule-coating sensitivity and specific selectivity. This study can be extended to sensing biological molecules such as proteins, DNAs, antibodies and antigens.

Temperature-dependent optical properties of gold thin films

Understanding the temperature dependence of the optical properties of thin metal films is critical for designing practical devices for high temperature applications in a variety of research areas, including plasmonics and near-field radiative heat transfer. Even though the optical properties of bulk metals at elevated temperatures have been studied, the temperature-dependent data for thin metal films, with thicknesses ranging from few tens to few hundreds of nanometers, is largely missing. In this work we report on the optical constants of single- and polycrystalline gold thin films at elevated temperatures in the wavelength range from 370 to 2000 nm. Our results show that while the real part of the dielectric function changes marginally with increasing temperature, the imaginary part changes drastically. For 200-nm-thick single- and polycrystalline gold films the imaginary part of the dielectric function at 500 0C becomes nearly twice larger than that at room temperature. In contrast, in thinner films (50-nm and 30-nm) the imaginary part can show either increasing or decreasing behavior within the same temperature range and eventually at 500 0C it becomes nearly 3-4 times larger than that at room temperature. The increase in the imaginary part at elevated temperatures significantly reduces the surface plasmon polariton propagation length and the quality factor of the localized surface plasmon resonance for a spherical particle. We provide experiment-fitted models to describe the temperature-dependent gold dielectric function as a sum of one Drude and two Critical Point oscillators. These causal analytical models could enable accurate multiphysics modelling of gold-based nanophotonic and plasmonic elements in both frequency and time domains.

Time-domain separation of optical properties from structural transitions in resonantly bondedmaterials.

The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.